Boundary friction force of tree frog’s toe pads and bio-inspired hexagon pillar surface

Abstract

In our daily life, there is growing use of polymer soft materials, such as rubber and silica gel. In some circumstances, their wet anti-skidding property plays an important role, e.g., rubber tires on car, windscreen wiper and rubber gloves. Its friction under the wet environment is difficult to study since the interaction effect of fluid, capillary effect and soft solid coupling together. As millions years of evolution, creatures have evolved some special soft skins to attach to all kinds of surfaces. Inspired from mother nature, as the toe pad of the tree frog can greatly attach to wet surface, it was used to study the wet friction theory of soft materials. The structure of its toe pad has been characterized by SEM images. The toe surface was constructed by arrayed pillars with a height of 5 μ m, circumcircle diameter of 10 μ m and gap of 1 μ m. About half of these pillars was hexagon, and the rest was pentagon, heptagon, and quadrangle. The friction of toe pads has been tested with consecutive sliding steps on dry substrates. Its friction increases with secretion decreasing and boundary friction formed just like the friction tests on human skin. Under the inspiration of tree frog’s toe pads, we designed and fabricated a bio-inspired soft surface (PDMS) with hexagon pillar array. With liquid between contact area decreasing from wet to dry condition, its friction shows a peak value, which is similar to the friction of tree frog’s toe pads. During boundary friction condition, the value can be 50 times higher than dry friction. The liquid between contact interface forms strong capillary force to rise friction as it can be regarded as extra normal force. Comparing to smooth surface, bio-inspired surface can maintain stable and long term boundary friction since the micro-pillars can separately deform to steady friction and the channels can hold more liquid than smooth surface. As hydrophilic hexagon pillar surface can quickly spread liquid through channels to create uniform thin liquid films between contact interfaces, it exhibits much better boundary friction property than hydrophobic one, whose boundary friction is almost 0 mN. By comparing the boundary and dry friction of four type of surfaces under different weight, the four dry friction coefficients display no change as common materials. However, the hydrophilic surface demonstrated the high boundary friction coefficient with little weight. This results from that capillary force between contact interfaces almost stays the same and only plays a great role when the external pressure between interfaces was small. With external normal force increasing, the affection of normal pressure induced by liquid capillary force will be weaken. The boundary of hexagon pillar surface is also different when sliding in the hexagon angle direction and side direction. This was in consistent with the distribution of tree frog’s toe pad channels. This study explains the mechanism of tree frog’s wet attachment ability. It also further expands the understanding of wet friction mechanism in soft materials. It will be useful for the designing of stronger wet attachable surface or preventing the stick-slide effect on linear guide way.